Ohm's Law

Voltage (volts) is the force that moves electrons, forcing a current.  Voltage can be compared to a tank of water elevated at a certain height (potential).   If the tank is placed low (low voltage), water will not flow very quickly (low current).  If the tank is raised to a higher location (higher voltage), the water will flow rapidly (high current).

Current (Amperes) is, in simple terms a measurement of how many electrons flow through a device.  In the water tank analogy, current would be water flow rate.

Resistance (Ohms) slows down current flow.  The higher the resistance of a circuit, the lower the current will be.  Resistance would be equivalent to pipe size.   If you have the water tank at a high level, but the pipe is very small in diameter (high resistance), not much water will flow.  If you use a big pipe (low resistance), then the water flow rate will be larger.

Knowing the relationships between voltage, current and resistance brings us to ohm's law:  "Current is proportional to Voltage divided by Resistance".  This equation can be manipulated to obtain any value knowing the other two.  For example, by measuring the voltage and current of a circuit, resistance can be calculated by dividing voltage by current.  When a circuit is open (disconnected), the resistance is infinite (zero current).  The formula also shows what happens when a circuit is shorted (resistance = 0): The battery will put out as much current as it can instantaneously (not a good sight).

DC vs. AC Circuits

On a DC circuit, current flows in one direction only.   Voltage can remain at a level or change, but it always has the same polarity.   A car's battery produces DC voltage.

AC circuits are a bit more complicated to understand.  The voltage supply reverses its polarity switching from positive to negative.  The current produced goes in one direction while the voltage is positive and then flows in the opposite direction when voltage is reversed.  AC circuits have a frequency associated with them.  The frequency (Hertz or Hz) is how many times per second (cycles) the current (and voltage) switch from positive to negative and back.  The higher the frequency, the faster the circuit will switch polarity.  AC voltage in the car is produced by the alternator, which is converted to DC voltage to charge the battery (even though some of the AC energy from the alternator remains in the electrical system, this is what causes alternator whine when the car is running).  The audio signal that comes from the head unit, gets amplified and drives the speakers is also an AC signal.

A Typical DC Circuit

Electrons flow in a circuit from the negative side of the battery to the positive side of the battery (that is why physicists will argue with the direction of the current in the circuit).  Engineers represent current in the opposite direction of electron flow, as in the diagram.  It does not matter what convention you follow for current direction.  The important thing to keep in mind is how much current flows through the circuit, and that you stick to only one of the models when analyzing a circuit.
For a circuit to have current, there has to be a path (i. e. wire) and a battery.  A circuit also has a resistance, which slows down flow of electrons.
If the path is broken, current can not flow.  The battery supplies the voltage.  The top portion of the circuit in a car is represented by the positive battery cable going to the fuse box and to all the accessories (radio, wipers, lights, etc).  Each accessory has a resistance.  As more accessories are added, the resistance drops, and more current flows through the circuit.  To save money, car manufacturers use the car metal for the bottom part of the circuit, instead of running a ground wire to every device.

Common Engineering Notations

To represent very high or low values, zeros or decimal points are represented by letters.  These are the most common used in car audio:

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